MLC and SLC NAND flash design tradeoffs

In the past few years, NAND flash has become the
storage solution of choice for a wider and wider range of
applications, spurred by a steadily decreasing price/bit achieved
by advances in multi-level cell (MLC) technology and progression to
smaller process geometries. NAND flash has enabled multiple markets
and today is gaining ground in laptop computers both in solid state
drives and various cache solutions.

Questions with fuzzy answers

This proliferation of NAND flash, however,
brought additional questions for system designers to consider. When
is it best to use MLC NAND vs. SLC (single level cell) NAND? What
exactly does that choice mean in terms of power, speed, error
correction requirements, and usable life?

The answers to these questions are not as
straightforward as one might expect, because of the rapid
progression of NAND flash from one geometry to the next, and a
proliferation of NAND manufacturers, each using slightly different
process geometries. For each process node, such as 56 or 43 nm, the
specifications for MLC vs. SLC NAND performance is different.

For those not familiar with the technology, MLC
NAND flash allows each memory cell to store two or more bits of
information, compared to one bit per cell for SLC NAND (see Fig. 1). MLC NAND has a larger capacity,
lower bit cost, and a decrease in several performance parameters.
However, for many applications, MLC NAND provides more than
suitable performance and longevity, and has helped make storage
more affordable. When given a choice, such as with high-performance
memory cards or USB drives, most customers have not been willing to
pay for the additional performance available from SLC NAND.

Fig. 1. MLC NAND uses four threshold
voltage levels within a cell to be able to store two bits of data
per cell, compared to two voltage levels and one bit per cell for
SLC NAND.

SLC NAND is typically used today for selected
applications that require either higher speed or greater endurance
to justify its price premium. According to market analyst firm
Web-Feet Research, MLC NAND shipments were approximately 78% of
total NAND shipments in 2007. The important thing is to choose the
appropriate performance level for the specific application to
balance the value/performance equation.

Design tradeoffs

For any specific process geometry, MLC NAND
writes data at approximately one half to one third the speed of SLC
NAND, requires a higher level of error correction, and has lower
write cycle endurance.

Power. The basic
circuitry for MLC and SLC chips is similar, and power consumption
for the two types of memory is approximately the same.

Speed. As noted
above, SLC and MLC NAND specifications change with each process
geometry, and can vary by manufacturer. As a rule of thumb, SLC
NAND is generally two to three times faster than MLC NAND. Toshiba
MLC NAND with a 4 Kbyte page size has a write speed of 4.5 Mbytes/s
using single plane capability, or 9 Mbytes/s in dual plane. The
write speed for the comparable SLC device is approximately three
times faster.

Error correction.
Just as hard-disk drives have some bad sectors, NAND works in a
similar way in that the controller maps around bad memory areas and
error correction (ECC) is used to correct bit errors. The industry
standard is to correct any bit error to a level comparable to that
of hard-disk drives, or 1 x 10-14, which means one bit
uncorrectable error every 1014
bits (12.5 Tbytes). For any given process node, MLC NAND requires a
higher level of ECC than SLC NAND to achieve this level of error
correction.

The industry provides embedded NAND solutions
with dedicated memory controllers optimized to provide ECC, wear
leveling and other NAND management capabilities (see Fig. 2) so designers do not need to
implement ECC themselves, or be concerned about the difference
between ECC requirements for MLC vs. SLC NAND.

Fig. 2. The industry has moved to
self-contained embedded NAND solutions with dedicated controllers
to handle ECC, wear-leveling, and other management
functions.

Endurance. MLC
flash has a different rating for the number of write/erase cycles
compared to SLC NAND flash. Today, most SLC NAND flash is rated to
have approximately 100,000 cycles, while industry specifications
for MLC range from 3,000 to 10,000. While that sounds like a huge
difference, specific application usage models are easier to
understand and evaluate.

As an example, based on our usage modeling,
MLC-based solid-state drives (SSDs) are a good fit for most mobile
computing applications, while SLC-based SSDs are better suited for
high-performance enterprise applications.

To even begin to reach a conservative endurance
limit of a 64-Gbyte MLC NAND-based SSD with wear-leveling
technology, a mobile user would have to write approximately 40
Tbytes of data over the expected five-year life of the drive.
That’s equal to approximately 22 Gbytes of new data per
day, every day.

With a 128-Gbyte drive, the wear would be spread
over a larger storage area, doubling the average daily limit to 44
Gbytes. Therefore the endurance limit is so far beyond the likely
usage of a typical mobile computer user that it may not be a
realistic cause for concern.

On the other hand, applications that involve
nearly constant writing might require SLC NAND. Examples may
include an enterprise solid state drive, or a personal video
recorder (PVR) that constantly records an hour of video, then
rewrites the previous hour’s images to record the most
recent video stream.

For example, a 4-Gbyte PVR, recording video at a
rate of 1 Mbyte/s, would write a total of 86.4 Gbytes/day, and
would use one complete write cycle every 1.1 hours. In this
example, the 100,000 rated write cycles of SLC NAND would last
approximately 12 years, more than enough to meet the typical 5-year
product life, while MLC NAND rated at 10,000 cycles would last only
1.2 years. ■